When the info type is MU, we still have the data from the TSF
overload words, so should decode that. When it's MU_EXT_INFO
we additionally have the SIG-B common 0/1/2 fields.

Also document the validity depending on the info type and fix
the name of the regular TB PPDU info type accordingly.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>